Palatal expansion appliances have long been used to expand a width of a narrow palate in a treatment process of malocclusions in dental clinics and are also used in the field of otorhinolaryngology to improve nasal breathing impairment and/or obstructive sleep apnea.
Hereinafter, the structure and operating principle of palatal expansion appliances that have been generally used to date are briefly described.
FIG. 1 schematically illustrates a palatal expansion appliance 10 according to the related art.
Referring to FIG. 1, the palatal expansion appliance 10 may include a screw rod 12, a pair of bodies 13, a pair of guide rods 14, two pair of tooth connecting rods 15, and a key 16.
The screw rod 12 has threads of two types respectively formed at one end portion and the other end portion thereof in opposite directions.
The screw rod 12 may include a keyhole formation portion 11 at a center portion thereof. A user inserts the key 16 into a keyhole kh and rotates the screw rod 12.
A plurality of keyholes kh are formed at a constant interval around the keyhole formation portion 11. For example, a total of two keyholes kh may be formed at an interval of 90° around the keyhole formation portion 11. The two keyholes kh may perpendicularly penetrate through the keyhole formation portion 11.
Each of the bodies 13 is coupled to the guide rods 14 which penetrate through the each of the bodies 13, and each of the bodies 13 is screw-coupled to the screw rod 12 to be moved along the guide rods 14 in a coupled state. In other words, each of the bodies 13 may include a part functioning as a nut. Each of the tooth connecting rods 15 is coupled to each of the bodies 13. Accordingly, each of the tooth connecting rods 15 transfers a force to teeth T of FIG. 2A, in which the force is generated as each of the bodies 13 moves. For example, each of the tooth connecting rods 15 is welded on each of the bodies 13.
Each of the guide rods 14 is coupled to the bodies 13 by penetrating therethrough, and stably guides each of the bodies 13 in an expansion direction according to rotations of the screw rod 12.
The key 16 may include a holding part 16a and an insertion part 16b. 
When the user holding the holding part 16a of the key 16 inserts the insertion part 16b into the keyhole kh and applies a force in a direction indicated by an arrow, the keyhole formation portion 11 rotates in the direction indicated by the arrow and accordingly the screw rod 12 rotates in the same direction as a rotation direction of the keyhole formation portion 11. In this state, each of the bodies 13 screw-coupled to the screw rod 12 moves in a direction away from the center of the keyhole formation portion 11. Accordingly, each of the tooth connecting rods 15 coupled to each of the bodies 13, for example, by welding, is moved in the direction away from the center of the keyhole formation portion 11 with each of the bodies 13.
FIG. 2A is a plan view illustrating a state in which the palatal expansion appliance 10 of FIG. 1 is installed around a palate PT or a roof of the mouth. FIG. 2B is a side view illustrating a state in which the palatal expansion appliance 10 of FIG. 1 is installed around the palate PT or the roof of the mouth. In FIG. 2A, the palatal expansion appliance 10 is arranged to be symmetrical to the left and right with respect to a median palatine suture (MPS). Each of the tooth connecting rods 15 of the palatal expansion appliance 10 is attached, for example, welded, to a band b encompassing the teeth T.
In the palatal expansion appliance 10 of FIG. 1, as a user rotates the keyhole formation portion 11 to rotate the screw rod 12, when each of the bodies 13 is moved, as illustrated in FIG. 2A, each of the tooth connecting rods 15 is moved in the direction indicated by the arrow, thereby transferring a force to the teeth T. The force transferred to the teeth T is re-transferred to the MPS via a gum bone (not shown) where the teeth T are implanted, so that the MPS is widened to the left and right. In the MPS widened as above, a bone is grown at a widened portion after about three months and the widened MPS is stably restored, thereby expanding the width of the palate PT.
The palatal expansion appliance 10 having the above structure and operation according to the related art, as described above, transfers a force to the teeth T at both sides, in particular, molar teeth at both sides, to apply the force to the MPS that is a target tissue to expand. In this state, if the MPS is not smoothly widened because resistance of the MPS is stronger than an expansion force, as illustrated in FIGS. 2B and 3, only molar teeth (MT) at both sides are inclined outwardly due to the expansion force. Accordingly, the initial treatment objective of widening a narrow palate may not be achieved, resulting in a failure of the treatment. The problem arises often particularly in the case of adults in which the MPS has already hardened, compared to adolescents in a growth phase. Accordingly, in the case of adults, in order to successfully expand a narrow palate, with the use of the palatal expansion appliance 10, a considerable operation may need to be performed on an upper jaw bone and the palate PT to reduce resistance of peripheral tissues, which may financially and physically burden a patient.
To prevent such a failure, there have been various trials to change the design of a palatal expansion appliance to transfer the expansion force of an appliance “fixed to both molar teeth (tooth-borne)” to the MPS. However, in the case of adults, the trials have obtained no great effect due to the resistance of the MPS that has already hardened, aside from the design of an appliance.
Accordingly, recently, there has been an increase in trials to directly apply a force to the MPS that is a target tissue to expand, not via the teeth, by directly fixing a palatal expansion appliance to bones around the MPS (bone-borne).
An example of such a trial includes a palatal expansion appliance 20 illustrated in FIG. 5A.
Referring to FIG. 5A, the palatal expansion appliance 20 may include a keyhole formation portion 21, a screw rod 22 having threads of two types formed on one end portion and another end portion thereof in opposite directions, a pair of bodies 23, a pair of guide rods 24, two pairs of tooth connecting rods 25, four bone screw holes 26, four bone screws 27, and a key (not shown, same as the key 16 of FIG. 1) for rotating the keyhole formation portion 21. Since each of the bone screws 27 is implanted into bone, the bone screws 27 are referred to as “bone screws”.
The palatal expansion appliance 20 of FIG. 5A is different from the palatal expansion appliance 10 of FIG. 1 in that the bodies 23 are provided with the bone screw holes 26 so that the expansion force may be directly transferred to an adjacent bone of the MPS by the bone screws 27, in addition to the teeth T. Portions of the palatal expansion appliance 20 of FIG. 5A that are not described below are considered to be identical to the portions of the palatal expansion appliance 10 of FIG. 1.
The screw rod 22 may include the keyhole formation portion 21 at the center portion thereof. A plurality of keyholes kh are formed at a certain interval around the keyhole formation portion 21.
Each of the bone screw holes 26 is formed at upper and lower sides of each of the bodies 23 in a direction perpendicular to a direction in which the palatal expansion appliance 20 expands. In detail, each of the bone screw holes 26 is formed at one end portion or another end portion of each of the bodies 23 in a lengthwise direction thereof.
An operating principle of the palatal expansion appliance 20 of FIG. 5A is similar to that of the palatal expansion appliance 10 of FIG. 1. As illustrated in FIG. 5B, the palatal expansion appliance 20 is installed on the teeth T. In detail, after bending each of the tooth connecting rods 25 corresponding to the shape of a palate so as to be fixed to the teeth T, a tooth band b encompassing the teeth T and corresponding to the shape of the teeth T is attached to the teeth T. Since the tooth connecting rods 25 and the tooth band b are welded to each other, a plaster mold is manufactured by making a negative imprint of the shape of a mouth of a patient using a dental impression material, the positions of the palatal expansion appliance 20 and the tooth band b are reproduced in the plaster mold and then the tooth connecting rods 25 and the tooth band b are welded to each other. The palatal expansion appliance 20 in the state in which the tooth band b is welded to the tooth connecting rods 25 is installed in the mouth of a patient by coating the tooth band b with a dental adhesive to fix the tooth band b to the teeth of a patient. Then, surface anesthesia and local anesthesia are performed on soft tissues and mucous membrane around the four bone screw holes 26 formed on the bodies 23 of the palatal expansion appliance 20. Next, each of the bone screws 27 is implanted into a bone through each of the bone screw holes 26. Also, as a result, the palatal expansion appliance 20 is fixed to each palatal bone and the teeth T. Next, when the keyhole formation portion 21 is rotated in one direction, each of the bone screws 27 implanted in the bone and each of the tooth connecting rods 25 connected to the teeth T via the tooth band b are moved in directions away from each other, that is, in directions in which the MPS expands. Accordingly, as the forces are transferred to the MPS via the teeth T and a gum bone into which the teeth T are implanted, and via the bone screws 27 implanted in the adjacent bone of the MPS, the MPS is widened. In other words, the palatal expansion appliance 20 described above is an appliance obtained by modifying the (tooth-borne) palatal expansion appliance dependent on teeth according to the related art, by which the expansion force is additionally applied to the bone through the four bone screws 27 implanted into the four bone screw holes 26 formed at the upper and lower ends of the bodies 23 of the palatal expansion appliance 20 in the lengthwise direction.
Although the above-described appliance that applies forces to the bone adjacent to the MPS together with the teeth is an improvement to reduce failure of the (tooth-borne) palatal expansion appliance according to the related art that applies a force to the teeth only, the appliance still has the following problems.                1) The palatal expansion appliance (see FIGS. 1, 2A, and 2B) has a width of the appliance itself for the screw rod, the guide rod, and the body needed to widen the appliance. No matter how wide the body expands, the screw rod and the guide rod inserted into the body should partially remain in the body to maintain firm coupling between parts of the appliances. Accordingly, assuming that the width of the palatal expansion appliance is 100%, an expandable width is limited to about 70% of the width of the appliance. However, as the width of a palate decreases, a wider expansion of the palate is needed to achieve an appropriate treatment object. In other words, although a palatal expansion appliance having a wider operation range for expansion is needed as a width of the palate decreases, a wide palatal expansion appliance may not be used for a narrow palate. In other words, it is difficult to use a palatal expansion appliance having a sufficient operation range for widening a palate with a palate having a relatively narrow width.        2) According to a recent change in a treatment method, bone screws are implanted into bones adjacent to the MPS to directly apply a force to the MPS, a portion having an appropriate thickness enduring an expansion force in the adjacent bones is located within a 3 mm offset to the left and right from a center line of the median palatine suture (see FIG. 4). Accordingly, considering the shape of the palatal expansion appliance according to the related art, the positions where the bone screws are implantable into bones within a 3 mm offset to the left and right from the center line of the median palatine suture through the bone screw holes formed in the bodies are only four positions at the upper and lower sides of each of the bodies in the lengthwise direction of the bodies.        
However, when the bone screws 27 are implanted at four positions only vertically in the lengthwise direction of the bodies of the palatal expansion appliance 20, if just one of the four bone screws 27 is loose with respect to the bone, the other bone screws 27 may not stably sustain the expansion force. In detail, when one of the four bone screws 27 is loose, it becomes difficult for the bone screw 27 located at the same side as the bone screw 27 that is loose to solely sustain the expansion force, and thus the coupling with the bone becomes loose within a short period of time. In other words, since the force needs to be uniformly transferred to the opposite portions that expand in order to have normal expansion, when only two bone screws 27 are provided at each side, if only one bone screw 27 is loose, it is difficult to stably perform the function of sustaining the expansion force. This is similar to a case in which, although an automobile having four wheels seems to be stable, if one of the wheels has a problem, it is difficult to stably perform a function. Accordingly, in order to reduce a bone screw failure risk in which the overall function of the appliance is deteriorated according to the failure of the bone screw, it is advantageous to implant three or more bone screws at each side.                3) When the palate to expand is very narrow, due to the width of the palatal expansion appliance 20 itself, it is difficult to fix the palatal expansion appliance 20 by using the bone screws 27 in a state in which the bodies of the appliance closely contact the palate, that is, the bone screw holes formed in the body sufficiently closely contact the palate (see FIG. 5C). In this state, as a lever arm between a point where the expansion force is generated by the screw rods 22 and a bone portion fixed by the bone screws 27 increases, a twisting force applied to the bone screws 27 increases so that the bone screws 27 may be highly likely to be taken off. In the present specification, the “lever arm” denotes a distance between a point where a force is generated and a point of action where the force acts.        4) Accordingly, in the “bone-borne” palatal expansion appliance 20 used in the case of adults having a harder MPS compared to that of adolescents, as illustrated in FIGS. 5B and 5C, to prepare for a case in which the bone screws 27 are loose, each of the tooth connecting rods 25 is fixed to the teeth T or the molar teeth MT by using the teeth band b. However, in order to fix the palatal expansion appliance 20 to the teeth T in the existing manner as described above, a patient needs to visit a hospital several times to manufacture a separate plaster mold and weld the palatal expansion appliance 20 to the plaster mold and reposition the palatal expansion appliance 20 in the mouth of the patient, the size of the appliance 20 increases such that eating food and managing oral hygiene are much inconvenienced. When the bone screws 27 are loose and lost, the teeth T finally receive the force and thus the teeth T are inclined outwardly and twisted, which is the disadvantage of the “(tooth-borne) palatal expansion appliance” of the related art.        5) When the palatal expansion appliance is wider than a narrow palate to the left and right, the body of the palatal expansion appliance does not completely fit to soft tissues of the palate in a lower side, the lever arm between the screw rods where the expansion force is generated and the bone screw insertion part increase as described above and thus the bone screw may not stably sustain the expansion force. Also, to reduce the above phenomenon, clinically, the palatal expansion appliance is arranged to be as close as possible to the palate and fixed thereto by using the bone screws. In this process, a certain part of the palatal expansion appliance excessively presses soft tissues of the palate and thus inflammation and pain may be highly likely to be generated.        6) In all palatal expansion appliances according to the related arts, in order to insert a key into a keyhole formed to rotate the keyhole formation portion, after accurately aiming at an inlet of the keyhole, the key should be inserted in the keyhole in a direction parallel to a direction of the keyhole key so that the key may be successfully inserted in the keyhole. Accordingly, an assistant trained with a method of effectively rotating the keyhole formation portion using the key is needed. The assistant needs to have a certain degree of understanding and proficiency about the structure and function of the palatal expansion appliance. However, a patient living alone may not find such an assistant and, even if an assistant is found, but the assistant lacks a degree of understanding and proficiency about the palatal expansion appliance, successful use of the palatal expansion appliance may not be guaranteed.        